Dialkyl tin dialkoxides are extremely useful as catalysts such as ester synthesis catalysts, carbonic acid ester synthesis catalysts, ester exchange reaction catalysts and silicone polymer or urethane curing catalysts. In particular, in addition to carbonic acid esters being used as additives such as gasoline additives for improving octane value and diesel fuel additives for reducing particles levels in exhaust gas, these useful compounds are also used as alkylation agents, carbonylation agents or solvents and the like during synthesis of polycarbonates, urethanes, pharmaceuticals, agricultural chemicals and other organic compounds, or as lithium battery electrolytes, lubricating oil raw materials and raw materials of deoxygenating agents for rust prevention of boiler pipes, thus resulting in dialkyl tin dialkoxides attracting attention as synthesis catalysts in particular. For example, International Publication No. WO 2003/055840 discloses a process for producing a carbonic acid ester comprising reacting an organometallic compound containing dialkyl tin dialkoxide with carbon dioxide followed by thermal decomposition of the formed addition product.
A conventionally known process for producing dialkyl tin dialkoxides comprises carrying out a dehydration reaction of dialkyl tin oxides and alcohols and removing the resulting low boiling point component that contains water from the reaction liquid (refer to, for example, U.S. Pat. No. 5,545,600, International Publication No. WO 2005/111049, Japanese Patent Application Laid-open No. 2005-298433, Journal of Chemical Society, 23 (1971), 3972, and Journal of the Chemical Society of Japan—Industrial Chemistry, 72, 7 (1969), 1543-1549). Processes for producing the dialkyl tin dialkoxides by the dehydration reaction of the dialkyl tin oxides and the alcohols are presumed to be equilibrium reactions accompanying dehydration as shown in the following formula (1) below:
(wherein R and R′ represent alkyl groups).
The above equilibrium is biased overwhelmingly toward the reactants, and is presumed to further contain successive dehydration reactions via tetraalkyl distannoxane as shown in the following formulas (2) and (3):
(wherein R and R′ represent alkyl groups);
(wherein R and R′ represent alkyl groups).
Although dialkyl tin dialkoxides are produced while removing water generated from each dehydration reaction outside the system in order to obtain the dialkyl tin dialkoxides at high yield, since this reaction is disadvantageous in terms of the reaction energy, the reaction is required to be carried out at a high temperature (for example, 180° C.) for a long period of time.
On the other hand, when dialkyl tin alkoxide compounds (such as dialkyl tin dialkoxides) are heated to, for example, about 180° C., variants are known to be formed such as trialkyl tin alkoxides having three alkyl groups on a single tin atom (see, for example, Journal of the Chemical Society of Japan—Industrial Chemistry, 72, 7 (1969), 1543-1549). Although it is not clear as to the type of reaction by which these trialkyl tin alkoxides are formed, it is presumed that alkyl groups are transferred, for example, and variants are formed by a disproportionation reaction as represented by the following formula (4) in the case said dialkyl tin alkoxide is a tetraalkyl dialkoxy distannoxane, or variants are formed by a disproportionation reaction as represented by the following formula (5) in the case said dialkyl tin alkoxide is a dialkyl tin dialkoxide:
(wherein R and R′ represent alkyl groups).
According to formula (4) above, a trialkyl tin alkoxide and a monoalkyl compound having a single alkyl group on a single tin atom are presumed to be formed as variants of tetraalkyl dialkoxy distannoxane. In actuality, since the inventors of the present invention have confirmed that trialkyl tin alkoxides and high boiling point tin components are included in variants of the tetraalkyl dialkoxy distannoxanes, the high boiling point tin component is assumed to correspond to the monoalkyl compound.
However, the structure of the highly boiling point tin component assumed to correspond to the monoalkyl compound has yet to be identified. Similarly, although variants presumed to be trialkyl tin alkoxides and monoalkyl tin alkoxides are formed from dialkyl tin dialkoxides, the structure of these variants presumed to be said monoalkyl tin alkoxides has not been identified.
The formation of such variants is also confirmed in, for example, the process of producing dialkyl tin dialkoxides as described above, and in processes for producing carbonic acid esters by reacting an organometallic compound containing dialkyl tin dialkoxides with carbon dioxide followed by thermal decomposition of the formed addition product.
Trialkyl tin alkoxides are known to have an extremely poor ability to produce carbonic acid esters in the production of carbonic acid esters by reaction between carbon dioxide and tin compounds (see, for example, Journal of American Chemical Society, 121 (1999), 3793). In addition, high boiling point tin components, included in said variants for which the structure has been unable to be identified, also have an extremely poor ability to produce carbonic acid esters in the production of carbonic acid esters by reaction between carbon dioxide and tin compounds (see, for example, Japanese Patent Application Laid-open No. 2005-298433).
In this manner, since variants do not demonstrate reactivity in the production of carbonic acid esters by the reaction between carbon dioxide and tin compounds, if variants are formed in the production process of said carbonic acid esters, variants of dialkyl tin alkoxide compounds having low activity accumulate when repeatedly using alkyl tin alkoxide compounds, thereby resulting in a decrease in the active form in the form of dialkyl tin dialkoxide compounds, and in turn causing a decrease in the reaction rate or yield of the carbonic acid esters. In such cases, although a method is typically employed, which comprises adding a small amount of fresh dialkyl tin alkoxide compounds in order to make the reaction rate and yield constant, if variants are left as is while simply continuing to add fresh dialkyl tin alkoxide compounds, there may arise the problem of a large amount of degradation products of low activity accumulating in the reaction system. In addition, even in the case of removing a portion of a mixture of alkyl tin alkoxide compounds containing variants of dialkyl tin alkoxide compounds from the reaction system while adding fresh dialkyl tin alkoxide compounds to maintain a constant concentration of dialkyl tin alkoxide compound in the reaction system, in addition to the removed variants of the dialkyl tin alkoxide compound becoming waste, since the active form in the form of the dialkyl tin alkoxide compound is also removed and discarded, significant problems occur in terms of costs and waste processing.
Several solutions to the above problems have been previously proposed (see, for example, International Publication No. WO 2004/014840 and International Publication No. WO 2007/097388). More specifically, International Publication No. WO 2004/014840 proposes a method used in the production of carbonic acid esters using dialkyl tin alkoxide compounds containing thermal denaturation products of dialkyl tin alkoxide compounds for separating trialkyl tin compound components from dialkyl tin alkoxide compounds containing said thermal denaturation products to prevent their accumulation in the reaction system. However, since high boiling point tin compounds having an unidentifiable structure contained in variants of the dialkyl tin alkoxide compounds are unable to be removed, the accumulation of variants of dialkyl tin alkoxide compounds cannot be completely prevented with this method.
In addition, the inventors of the present invention disclosed a method for separating and recovering products formed via dialkyl tin alkoxide compounds in the form of dialkyl tin dialkoxides by preliminarily reacting a dialkyl tin alkoxide compound and variants of the dialkyl tin alkoxide compound extracted from the reaction system with an alcohol and/or carbonic acid ester (see International Publication No. WO 2007/097388). According to this method, the problem of the active form in the form of the dialkyl tin alkoxide compound being discarded with the variants is resolved, enabling only variants of dialkyl tin alkoxide compounds to be selectively discarded. However, since variants of the dialkyl tin alkoxide compounds cannot be reused, the problems of costs and waste processing remain.
On the basis of this background, there is a need for the development of a technology that allows variants of dialkyl tin alkoxide compounds to be regenerated into an active form in the form of dialkyl tin alkoxide compounds and be reused in the production of carbonic acid esters.
Proportionation reactions, which are the reverse reactions of the above-mentioned disproportionation reactions, are used as a method to obtain dialkyl tin compounds from mixtures of two types of compounds having different numbers of alkyl groups on the tin atom. For example, in the case of tin halide compounds, dialkylchloro tin is formed by a proportionation reaction between trialkylchloro tin and alkyltrichloro tin as represented by the following formula (6) (see, for example, Japanese Patent Application Laid-open No. H4-81999).

As previously described, disproportionation reactions that denature dialkyl tin alkoxide compounds into trialkyl tin alkoxides and monoalkyl tin compounds by a disproportionation reaction are advantageous in the case of tin alkoxide compounds, and it is difficult for the reverse reaction in the form of the proportionation reaction to occur. On the other hand, proportionation reactions are advantageous in the case of tin halide compounds, allowing the obtaining of dialkyldichloro tin from trialkylchloro tin and alkyltrichloro tin.
Several methods have been previously proposed for the production of alkyltrichloro tin (see, for example, Japanese Patent Application Laid-open No. H4-81999 and Japanese Patent Application Laid-open No. S44-8489). More specifically, Japanese Patent Application Laid-open No. H4-81999 discloses a method for producing alkyltrichloro tin using the proportionation reaction as described above using a mixture of tetraalkyl tin and tetrachloro tin at a specific ratio. Japanese Patent Application Laid-open No. S44-8489 discloses a method for producing alkyltrichloro tin by reacting alkane stannonate and hydrogen chloride. However, a technology is not yet known for producing alkyltrichloro tin compounds by using as raw materials variants of dialkyl tin alkoxide compounds.
On the other hand, reactions in which trialkyl tin acetoxides and alkyl tin acetoxide oxides are formed by reacting variants of dialkyl tin alkoxide compounds with acetic acid have been disclosed as reactions of variants of dialkyl tin alkoxide compounds (see, for example, Journal of American Chemical Society, 121 (1999), 3793). However, a method is not yet known for producing dialkyl tin alkoxide compounds by the proportionation reaction between trialkyl tin acetoxides and alkyl tin acetoxide oxide compounds.
On the basis of the above, since the development of technologies for regenerating variants of dialkyl tin alkoxide compounds into active forms in the form of dialkyl tin alkoxide compounds has yet to be achieved, the problems of costs and waste processing in the production process of carbonic acid esters remain unsolved.